A photobioreactor is a system in which biological interactions, that it is sought to control by controlling the culturing conditions, take place in the presence of light energy. Within it, a biochemical photosynthesis reaction takes place with the aim of producing plant biomass from photosynthetic microorganisms, light, carbon dioxide gas and a minimum amount of mineral elements. During the mechanism of photosynthesis, most of the organic components are produced (carbohydrates, lipids and proteins).
Microalgae, which are photosynthetic under solar radiation, make it possible, in aquaculture, to produce a biomass of which the composition depends on the species selected, and which can reach, in terms of energy value, according to estimations, twenty to one hundred times that of plants in the ground. There are a few thousand genera of microalgae divided up into species, some of which represent a broad group potentially providing compounds that can be exploited as biofuels or in the cosmetics, pharmacy or even food-processing fields.
Some species are rich in lipids; it is possible to extract the oils (triglycerides) therefrom, giving more or less directly a biofuel. The residues can thus be exploited, for example by fermentation producing bioethanol. Lipid-rich microalgae thus become a technical solution to the energy problems related to the depletion of oil fields and to the replacement of starting materials of fossil origin with renewable materials. Furthermore, the production of algae is accelerated by sparging with CO2 derived, for example, from a polluting industry such as a fuel-burning power station, thus avoiding the immediate discharge of this greenhouse-effect gas. Microalgae also constitute an alternative to the obtaining of biofuels from plant crops (for example rapeseed, beetroot, wheat) which require large cultivatable surface areas. According to American scientists, some microalgae would be capable of synthesizing 30 times more oil per hectare than the terrestrial plants used for the production of biofuels. Moreover, the use of this starting material as a source of biofuels avoids the problems of seasonality and supply which are characteristic of the use of terrestrial plants.
Composed essentially of proteins, some species have a good food precedent since they are rich in vitamins, in polyunsaturated fatty acids and in trace elements. They find applications in agriculture and horticulture through the use of algal extracts which perform not only the role of a fertilizer, but also of a crop accelerator and protector.
Other species represent a productive potential applicable in health fields, for the synthesis of medicaments of biocosmetics from extracts of substances with biological or therapeutic activities, and in the food-processing industry through the extraction and production of pigments, natural dyes or gelling agents. Microalgae can constitute potential sources of molecules that are difficult to obtain by chemical synthesis.
Microalgae can be directly involved in the production of new and renewable energy, in particular the production of biofuels such as hydrogen. Some species can produce hydrogen under the action of the hydrogenase-type enzymes present.
The culture of microalgae offers an advantageous alternative for the treatment of wastewater (municipal, industrial or agricultural effluents). It allows a tertiary biotreatment coupled with the production of potentially exploitable biomass. As a purifying agent, microalgae play various roles such as the simultaneous elimination of nutritive salts (NH4+, NO3−, PO43−), the purification of secondary effluents through the production of oxygen which is used by the bacteria to degrade residual organic compounds, or a bactericidal action which reduces the survival of the pathogenic microorganisms contained in the secondary effluents. This technology has the advantage of being based on the principles of natural ecosystems and is therefore not dangerous to the environment.
The growth of microalgae can be carried out at the surface of solar tanks which are open or under greenhouses, which constitutes photobioreactors of limited investment. However, this technology is limited to the use of robust species which can withstand variations in temperature or in sunshine, or viruses. The production of biodiesel from the resulting biomass has not proven to be economically competitive compared with oil products.
For a more stable or more intensive production, closed solar photobioreactors, with transparent walls, are used. This equipment generally has a planar geometry (flat photobioreactors) or a cylindrical geometry (tubular photobioreactors). It is also possible to use photobioreactors with a more particular geometry, such as structures of the “H”-shaped or “I”-shaped hollow profiled element type, for example.
A flat photobioreactor is composed of two transparent parallel panels, of varying surface areas, and between which lies a thin layer of culture which is a few centimeters deep.
A tubular photobioreactor is composed of one or more transparent tubes, of varying diameters and lengths, of various configurations, and within which the culture can circulate. There are many configuration variations:                a wide vertical tube forming a column,        two tubes of different diameters arranged one inside the other, forming an annular chamber,        a tube placed on the ground and of moderate diameter but of long length, arranged in the form of a coil,        a tube of small diameter and of long length coiled helicoidally around a tower,        several tubes of small diameter arranged in parallel and vertically.        
The particularity of a photobioreactor stems, in addition to the usual needs common to all bioreactors, from the need to supply the microorganisms to be cultured, in particular the microalgae, with a photon energy, this provision being indispensable for the implementation of photosynthesis. From a technological point of view, the chamber of the system must therefore be transparent and be designed so as to supply a sufficient light intensity for the microalgae.
In order to meet this requirement, photobioreactors are generally made of a material which has a high transparency in the visible range, such as glass or polycarbonate (PC).
However, the use of glass, which in addition must be of high purity, has several drawbacks: it is a heavy, expensive, rigid material which breaks easily and which is difficult to machine. Glass also exhibits light scattering which is not as good as methacrylic polymers such as PMMA. Its use limits the choice of the geometry of the reactor: arrangements in the form of loops or coils are difficult to produce, tubes connected to one another by piping or connectors are sources of leakage; in order to maintain correct access of the light, the reactors must be cleaned very frequently owing to the formation of a biofilm on their walls. Optimization between a long tube length, a small surface area on the ground and the accessibility of light to the culture medium proves to be difficult.
Polycarbonate is a product which is not very UV-resistant over time, and the light transmission of which is less than that of PMMA. Moreover, for photobioreactors which require walls of large thickness, PC becomes very fragile. Finally, PC has a low chemical resistance to washing products such as hydrochloric acid, bleach and ozone.
The geometry of the reactor should as far as possible favor a high illuminated surface area to culture volume ratio, while at the same time limiting the ground surface area and remaining appropriate for the objectives of desired biomass concentration and physiological needs of the microorganisms cultured. A hydrodynamic/regulation balance is also to be sought in order to ensure sufficient mixing and a control of the parameters such as consumption of CO2, oxygen given off, or temperature.
In any event, the access to light should be optimized such that the light really available is substantial and uniform within the culture. Different variants are used, for example inclining flat reactors so as to improve the use of solar irradiance, placing the reactors on reflecting surfaces in order to increase radiation incidence by reflection, having artificial light sources in tubes at the heart of the culture medium, etc.
Photobioreactors give rise to many publications; for example, the following documents describe various configurations for carrying out photosynthesis reactions using microorganisms: EP 112 556; EP 239 272; WO 99/20736; WO 00/12673; WO 00/23562; FR 2 907 311; WO 07/025145; WO 08/040828; US 2005/064577.
Regarding the nature of the transparent materials used to produce the culture zones, or reaction tubes or chambers for the microorganism photosynthesis, the prior art proposes various materials among plastics such as polyethylene (PE), polycarbonate (PC), poly(methyl acrylate) (PMA), methacrylic polymers such as poly(methyl methacrylate) (PMMA), cellulose acetate butyrate (CAB), polyvinyl chloride (PVC), polyethylene terephthalate (PET), glycol-modified polyethylene terephthalate (PETG), or nonplastic materials such as glass.